A new available large spectrum block for fourth generation (4G) evolution radio systems and fifth generation (5G) radio systems is expected to be taken from a higher frequency band (e.g. above third generation (3G) radio systems) with most of the available spectrum unpaired. Time division duplex (TDD is promising for spectrum in the higher frequency band due to no requirement for a paired frequency band, channel reciprocity for MIMO and cost-effectiveness. 4G evolution should aim to enable higher performance, such as higher data rate and lower latency, which requires continued improvement for TDD.
In the 3rd Generation Partnership Project (3GPP) TS 36.211 standard, three radio frame structures are supported. Frame structure type 1 (FS 1) is applicable to frequency division duplex (FDD) only, frame structure type 2 (FS 2) is applicable to time division duplex (TDD) only, and frame structure type 3 (FS 3) is applicable to licensed assisted access (LAA) secondary cell operation only.
With FS 2 for TDD, each radio frame of length 10 ms consists of two half-frames of length 5 ms each. Each half-frame consists of five subframes (SFs) of length 1 ms. Each subframe (SF) is defined by two slots of length 0.5 ms each. Within each radio frame, a subset of SFs is reserved for uplink (UL) transmissions, and the remaining SFs are allocated for downlink (DL) transmissions, or for special SFs, where the switch from DL to UL occurs. Downlink refers to transmissions from a network node, such as a base station, to a wireless device, such as a mobile phone. Uplink refers to transmissions from the wireless device to the network node.
As shown in Table 1, copied from the third generation partnership project (3GPP) standard, 3GPP TS 36.211, version 13.0.0, seven different DL/UL configurations are supported for frame structure 2 (FS 2). In Table 1, “D” denotes a DL SF, “U” denotes an UL SF, and “S” represents a special SF. Configurations 0, 1, 2, and 6 have 5 ms DL-to-UL switch-point periodicity, with the special SF existing in both SF 1 and SF 6. Configurations 3, 4 and 5 have 10 ms DL-to-UL switch-point periodicity, with the special SF in SF 1 only.
TABLE 1DL/ULDL-to-ULConfig-Switch-pointSF numberurationperiodicity012345678905msDSUUUDSUUU15msDSUUDDSUUD25msDSUDDDSUDD310msDSUUUDDDDD410msDSUUDDDDDD510msDSUDDDDDDD65msDSUUUDSUUD
A special SF is split into three parts: a DL part (DwPTS), GP (Guard Period) and an UL part (UpPTS). The DwPTS with a duration more than 3 symbols can be treated as a normal DL SF for data transmission. However, the UpPTS is not used for data transmission due to the very short duration. Instead, UpPTS can be used for channel sounding or random access. In 3GPP TS 36.211, version 13.0.0, a set of DwPTS/GP/UpPTS configurations (lengths of DwPTS/GP/UpPTS in symbols) is supported as shown in Table 2. In Table 2, CP refers to the cyclic prefix. Table 2 shows the lengths of DwPTS and UpPTS in symbols.
TABLE 2Special SFNormal CP for DL and ULExtended CP for DL and ULconfigurationDwPTSGPUpPTSDwPTSGPUpPTS0310138119419212103110113112111014121137253929126932100271022552811129662
Typically, the DL/UL configuration and the configuration of the special SF used in a cell are signaled as part of the system information, which is included in a system-information block 1 (SIB1) and broadcasted every 80 ms within SF 5. The DL/UL configuration in a cell may vary between frames to adapt to the traffic needs.
Packet data latency is one of the performance metrics that vendors, operators and end-users (via speed test applications) regularly measure. Latency measurements are done in all phases of a radio access network system lifetime, when verifying a new software release or system component, when deploying a system and when the system is in commercial operation. The latency is determined by several factors with two of them related to TDD frame structure, frame alignment and hybrid automated repeat request (HARQ) round trip time (RTT). An enhanced LTE-TDD frame structure can provide better frame alignment for shorter TTI and shorten the HARQ RTT.
Due to the existing frame structure design, the HARQ RTT is long in existing TDD systems, because downlink HARQ-ACK feedback can be performed only in an uplink subframe and uplink HARQ-ACK feedback can be performed only in a downlink subframe and DwPTS symbols of a special subframe.
Shorter latency than previous generations of 3GPP radio access technologies (RATs) was one performance metric that guided the design of Long Term Evolution (LTE). LTE is also now recognized by end-users to be a system that provides faster access to the Internet and lower data latencies than previous generations of mobile radio technologies.
Packet data latency is important not only for the perceived responsiveness of the system, it is also a parameter that indirectly influences the throughput of the system. Hypertext transfer protocol/transmission control protocol (HTTP/TCP) is the dominating application and transport layer protocol suite used on the Internet today. According to HTTP Archive (http://httparchive.org/trends.php) the typical size of HTTP based transactions over the Internet are in the range of a few 10 s of Kbytes up to 1 Mbyte. In this size range, the TCP slow start period is a significant part of the total transport period of the packet stream. During TCP slow start, the performance is latency limited. Hence, improved latency can improve the average throughput for these types of TCP-based data transactions.
Radio resource efficiency could be positively impacted by latency reductions. Lower packet data latency could increase the number of transmissions possible within a certain delay bound; hence higher Block Error Rate (BLER) targets could be used for the data transmissions freeing up radio resources, potentially improving the capacity of the system.
One area to address when it comes to packet latency reductions is the reduction of transport time of data and control signaling, by addressing the length of a transmission time interval (TTI). In LTE release 8, a TTI corresponds to one SF of length 1 millisecond. One such 1 ms TTI is constructed by using 14 orthogonal frequency division multiplex (OFDM) or single carrier frequency division multiple access (SC-FDMA) symbols in the case of normal cyclic prefix and 12 OFDM or SC-FDMA symbols in the case of extended cyclic prefix. In LTE release 13, a study item has the goal of specifying transmissions with TTIs that are much shorter than the LTE release 8 TTI.
The short TTI is defined to have any duration in time and comprise resources on a number of OFDM or SC-FDMA symbols within a 1 ms SF. As one example, the duration of the short TTI may be 0.5 ms, i.e., seven OFDM or SC-FDMA symbols for the case with normal cyclic prefix. Another example is a TTI of only two OFDM or SC-FDMA symbols.
Based on the existing FS 2, as given in 3GPP TS 36.211, UL data and control information may only be transmitted in UL SF, and downlink transmission is only possible in DL SF and DwPTS of the special SF. Therefore, the delay for a granted UL data transmission will depend on when the next UL SF occurs, and the delay for a granted DL data transmission will depend on when the next DL SF or DwPTS occurs. The latency will thus depend on the frame alignment in TDD. The hybrid automated repeat request (HARQ) timing for UL and DL transmissions also depends on the DL/UL configurations, which in turn has an impact on HARQ round-trip-time (RTT).
Based on the existing FS 2, the latency due to frame alignment and HARQ RTT for TDD is much longer than that for frequency division duplex (FDD). Even with shortened TTIs, the latency in TDD cannot be scaled linearly proportional to the TTI length, and it is limited to the additional waiting time due to the DL/UL configurations.